You can spend hundreds of dollars on 'how to' photography
books. They
are filled with beautiful color photographs. They talk about composition
and lighting in great detail. But, until you know and understand the
real nuts and bolts of photographic theory, you're only shooting in the dark,
so to speak.

As digital photography becomes more and more popular,
the basic concepts of photography become more and more obscured by the
technology, but none the less important. The relationship between aperture, shutter speed and media
sensitivity still are the basic building blocks of image capture. For
the purposes of this discussion I will be using the term 'media' to describe
the image recording material used in the photographic process, whether electronic
(CCD) or photochemical (film). Under "normal" conditions any reasonably
sophisticated camera will produce acceptable results with no intervention from
the photographer. It's that 10% of situations where intelligent intervention
is required that can turn an automated failure into a triumph of photographic
skill.

The easiest to understand is shutter speed. This is the
amount of time the media is exposed to light. The amount of light
reaching the media in 2 seconds is twice as much as reaches the media in
1 second. Typically, shutter speed is measured in fractions of seconds. A
typical camera will allow variable shutter speeds from 1 to 1/1000 of a
second in 1 "stop" increments. The scenario might look like
this: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. The
little variances in the 'one half' rule is for convenience and does not
significantly impact the overall exposure. Shorter shutter
speeds allow less light to pass than longer shutter speeds.

Why do we care about what shutter speed we use? Fast moving action
will cause blur with a long shutter speed. Waterfalls and ocean waves
look really cool at very long exposures. Dark conditions may require
longer shutter speeds to allow sufficient light to pass to the media.

What factors determine whether I should pay attention
to the shutter speed? The
amount of action in a scene and your vision for its effect will be an important
factor. If motion is not a factor, look to other issues for clues
to exposure priorities.

Aperture

The aperture is easy to understand, but those numbers! 1.4,
1.8, 2.8, 5.7, 22, What's that all about? It's actually easy to understand,
and a little math will help explain it in detail. Aperture is expressed
as a ratio of the diameter of the light admitting opening to the focal
length of the lens. A 50 mm lens with a light opening diameter of
50 mm would be rated at 1:1, and expressed as f1. This 50 mm opening
would have an area of 1963 square millimeters (pi * r2 or
3.14 * 25 * 25). To reduce the amount of light passing through the opening
by one half, we would reduce the area of the opening by half to 981. Divide
by pi (= 312.2619983463), take the square root to determine the radius
(= 17.67093654412), multiply by 2 to find the diameter (= 35.34187308824). Then,
divide the focal length by the diameter and we discover a ratio of 1:1.4
or, f1.4. This process continues all the way DOWN the scale. Smaller
apertures are referred to as stopping down, even though the number becomes
larger. The sequence - 1, 1.4, 2, 2.8, 4, 5.7, 8, 11, 16, 22, 32...
each allowing half as much light to pass through the opening over a given
period of time.

Why do we care about aperture? Large apertures require careful focusing
but let in lots of light. Very small apertures have a greater 'depth
of field', which we will talk about in a little more detail later, but
restrict the amount of light and require longer exposure or more sensitive
media.

What factors determine whether I should pay attention
to the aperture? Again,
your subject makes all the difference. Are you more concerned with
how much of your picture is in focus, or how action is rendered in the
final image.

Media Sensitivity

In a film camera, media sensitivity is indicated
by the ISO rating of the film. Again, an exponential scale is used. Film with
an ISO rating of 200 is twice as light sensitive as film with a rating
of 100. 400 speed film is twice as sensitive as 200. Many digital
cameras have selectable sensitivity control and rate that sensitivity on
an ISO scale for convenience. I do not know the standard on which
the ISO scale is based, but it can be understood in a purely theoretical
sense. Whether the scale is a referential value or purely arbitrary
is not important.

Why do we care about media sensitivity? With film, faster or more
sensitive films are 'grainy', producing lower resolution. Slower
film has a finer grain, thus higher resolution. Digital equipment
is overcoming some of these issues, but even the current 'mega-pixel' cameras
still fall short of the resolution and color rendering capabilities of
even the fastest analogue films.

What factors determine the media sensitivity to use? With film,
grain must be balanced against speed depending on the size of the enlargement,
or sensitivity against the available light on the subject. With electronic
media it is important to understand the tradeoffs between image quality
and light sensitivity.

Stop

A "stop", as alluded to previously, is photo-technical jargon
for an adjustment that allows twice or half as much light to reach the
media. Manual camera equipment has mechanical detents, or stops,
that allow the photographer to 'feel' the adjustments without needing to
divert his gaze from the viewfinder. A skilled photographer could
quickly adjust his equipment to meet the demands of his subject without
the aid of built-in light meters and other 'helpful' automations. Once
the amount of light falling on a subject is known, adjustments may be made
to accomplish various photographic goals. Both the shutter speed and aperture
are usually adjustable in one stop increments. Double check your
equipment. Some cameras have aperture 'clicks' on the 1/2 stop
as well.

Light Meters

A light meter is a very handy (but not absolutely
necessary!) piece of photographic equipment. OK, ok, there are some situations
where achieving an appropriate exposure would be extremely difficult to
impossible without one, but for general purpose outdoor or indoor flash
snap shooting a light meter is only a convenience. Ok, an extreme
convenience.

There are two basic types of light meters -- reflected-light
and incident-light. Reflected
light meters come in two basic flavors -- average and spot. A spot
meter measures the light reflecting off a very small portion of the subject. The
average meter takes all the light from the scene and gives you an average.

Through years of statistical analysis it has been discovered that
the average scene reflects 18% of the light that falls on it. This
18% is what reaches your camera. Your light meter is programmed to
know that given a certain sensitivity of media and a given aperture, a
specific shutter speed is necessary to correctly register an image at an
average reflectivity of 18%. Herein lies the rub. Not every
scene reflects exactly 18%. Let your light meter do all the work
on the slopes during a day of snowboarding and you may be a bit disappointed
in the results. I recommend memorizing, yes, memorizing that little
chart printed on the inside of the film box. Before you release the
shutter, double check to see that the exposure being registered by your
light meter is reasonable. If in doubt, bracket. Bracketing
means take a couple more pictures, manually overriding the exposure a stop
or two, correcting what might be a mis-interpretation of the reflectivity
of your scene on the part of your light meter.

The spot meter measures specific portions of the scene,
allowing the photographer the opportunity to gather more specific information
about lighting conditions and, in capable hands, produce a more accurately
calibrated exposure. This
method takes more skill and time. Learn more about Ansel Adam's 'zone'
system and you'll want a spot meter of your own.

The incident-light meter measures the actual light
falling on the subject. Thus,
it would seem possible to produce accurate exposures every time. This
is entirely true, except say, when your subject is a black bear standing
in a dark forest. I've heard if you ask politely, a bear will stand
quietly by while you take your readings and won't eat you until you turn
to leave. (The incident light meter is most effective in the studio
where subjects are less likely to eat their photographer.)

Depth of field

The distance between the closest object and the
farthest that are in acceptable focus is considered the 'depth of field'. Depth
of field increases with smaller apertures (bigger f numbers) and
decreases with larger apertures (smaller f numbers).

Depth of field is either a by-product of shutter
priority automation or the primary objective of aperture priority automation. Many
times an exposure is a delicate balance between the desired depth of
field and a shutter speed necessary to arrest undesired motion.

Some rules of thumb - Wider angle lenses (shorter
focal length) have an inherently greater depth of field at a given aperture
than longer lenses. Closer
focus results in shallower depth of field than more distant focus.

Depth of field can be used to great effect. Use a large aperture
to accentuate a the main subject in an image, leaving the background out
of focus. Stop down to the smallest aperture possible for broad panoramas
with foreground details preserved intact. Depth of field is an important
photographic tool.

Exposure

In summary, exposure is the delicate balance among
a number of factors. At times, the best image possible is a tradeoff between
the ideal and the practical. Understanding all the options and
their implications is all that is necessary to make informed decisions
about the final image.

Advanced Stuff:

Flash photography

Flash photography seems straight-forward enough -
Shine a light on your subject and take a picture. That's what it would seem, anyway. And,
actually, the advent of the electronic 'strobe' flash has revolutionized
artificial light photography. But, there are still things to be learned. First
of all, here's how it really works - Open the shutter. Turn
on the light. When your subject is adequately illuminated, turn off
the light. Close the shutter. And, it all happens with the
push of a single button. Automatic flash exposure takes a lot of
the guesswork out of the process. But, what if conditions aren't
perfect?

Your subject sits in a large space against a very
dark background. Your
automatic flash floods the area with light, draining every last electron
from its capacitor. Its sensors still believe the image is underexposed. Your
picture reveals quite the opposite - the subject is grossly overexposed,
beyond retrieval. You need to calculate a manual exposure. Here's
what you do. Get out your flash's manual. Read the fine print
back in the back where it tells what the 'guide number' is. This
number tells you how much light your flash puts out. Sort of. The
guide number is also based on a film speed - the ISO rating.

A flash with a guide number of 45 meters with ISO 200 film means that
a properly exposed image would be achieved at a distance of 45 meters at f1.0. To
calculate other exposures for other distances, divide the guide number
by the distance to the subject yielding the appropriate aperture or f number.

That's simple enough. What if you're not shooting ISO 200
film? A new guide number must be calculated. That formula is:

Throw a calculator in your camera bag and get over
it. You're going
to need it anyway if you get into...

Photomacrography

I can hear it now - 'You mean macrophotography,
don't you?' - No, I don't. The term 'macro' as defined by Webster
- of, involving,
or intended for use with relatively large quantities or on a large scale. Thus,
macrophotography is photography with huge cameras using giant sheets of
film. A long time ago Kodak was roaming the country with a camera
that produced three foot by two foot slides that were enlarged to 30 feet
by 20 feet and displayed with backlighting in Grand Central Station in
New York. That is macrophotography on a grand scale. Photomacrography,
on the other hand, takes very small subjects and enlarges them life size
or larger. Knowing what we do about depth of field, it's easy to
understand in extremely close quarters a very tiny aperture will be necessary
to focus a suitable portion of the subject unless it is perfectly flat.

The terms are similarly confused in the micro world as well. Micro
photography produces images that require a microscope to view, as in spy
cameras and silicon chip production. Photomicrography is the process
of taking pictures through a microscope.

As if this weren't challenge enough, we start encountering
another phenomenon of photographic theory. Previously, we were taking light reflecting
off a large object and concentrating it into a smaller point. All the light
(well, that magic 18% anyway) that was reflecting off of the entire side
of your house was focused down onto a tiny piece of film barely an inch
square, or a CCD the size of your fingernail. Now, we are taking
a very small subject and spreading that light around a much larger area,
sometimes larger than the object reflecting the light itself.

You guessed it. Another formula. This one allows you to correct
for the amount of extra light necessary to properly expose the image on
the media. Now, unless you have rather sophisticated digital equipment,
this stuff is just for fun, right? Here we go...

Exf = (M+1)f

There, that wasn't so bad was it? OK, a little
more detail...

Exf is the effective aperture
M= image size / subject size f is the aperture marked on the lens

M is the most difficult number to derive but basically
means 'magnification'. Measure
the size of the object on the film and divide it by the size of the actual
subject. The only problem with this approach is by the time the image
is on the film so you can measure it, it's too late and underexposed. So,
if you're really fussy, you can try this version:

M= focal length / (subject distance - focal length)

Many advanced cameras have a mark on the top plate
of the body marking the exact location of the film plane. This is really handy for calculating
effective aperture for close-up photography. A tripod is really handy
for making measurements. (Toss a tape measure in your camera bag along
with the calculator.) A simple estimation of object size through the
viewfinder may be adequate. The image visible in the viewfinder in
the standard 35mm single lens reflex camera is approximately 35mm wide.
A well informed estimation of size is probably adequate.

So, what does this all mean? If the image on
the film is one inch long, and the subject is also an inch long, then
M=1 and the effective aperture of a lens at f2 is (1+1)*2 = 4. In other words at
a magnification factor of 1:1, you're losing 2 stops of your expected exposure. If
you were trying to improve your depth of field by shooting at f16,
then expect to need to add enough light to expose the media as if you were
shooting at f32. If your macro lens stops down to f32,
then plan on flooding your subject with enough light to adequately expose
a normal image at f64! Be careful. A large flash at
close range is capable of incinerating small insects and igniting fabric
and paper.

Remember, cameras don't take pictures. People do. You are responsible
for the final outcome. It's ignorance of the theory and techniques
that create bad pictures.